1. Field of the Invention
This invention relates generally to a system and process for the treatment of wastewater and, in particular, to such a system and process utilizing sequencing batch reactors.
2. Description of Related Art
In the early history of wastewater treatment by microorganisms, the wastewater was often batched and treated by various processes of agitation, aeration or the like. With the amount of wastewater to be treated increasing in volume and in impurities, batch treatment became fairly labor intensive and was eventually substantially replaced by continuous wastewater treatment processes in the 1920's and 1930's.
However, with the relatively recent innovation of computers which can be programmed to control valves, motors, etc. in the wastewater treatment process system, batch reactors and other periodic processes again appear to be a viable alternative and offer attractive advantages over continuous processes.
A sequencing batch reactor system generally incorporates a series of batch reactors, usually two or more, which use a sequence of steps to treat wastewater. Each of these reactors retains a certain amount of activated sludge and allows for the removal of excess sludge. The activated sludge contains microorganisms, which assist in the breakdown of waste materials when provided with adequate oxygen levels.
Each reactor in a conventional sequencing batch reactor system operates in a cyclical process. During the cycle for each particular reactor, the reactor must complete the process of treating a batch of wastewater. The batch wastewater treatment process includes a fill period, a react period, a settle or quiescent period and a decant period. An idle period may also be included in the treatment process.
In a conventional sequencing batch reactor (SBR) process, during the fill period, wastewater is introduced into a batch reactor and the wastewater level in the filling reactor rises. The fill period can be further divided into an anoxic fill period and an aerated fill period. During the anoxic fill period, wastewater is introduced into the batch reactor without aeration, and during aerated fill, the wastewater already introduced into the batch is aerated while the reactor continues to fill, thereby providing oxygen to the microorganisms in the activated sludge.
At the end of the fill period, incoming wastewater is diverted to another batch reactor, which then begins its cycle. The just filled reactor then enters the react period wherein the wastewater contained in the reactor is aerated for a predetermined time period, or until the desired level of treatment is attained. Aeration of the contents of the batch reactor results in the mixing of the activated sludge and the wastewater as well as the introduction of oxygen into this mixture. The introduction of oxygen into the mixed wastewater and sludge is required by the microorganisms contained in the sludge to effect the decomposition of various wastewater components, including biodegradable organic matter.
At the end of the predetermined aeration period, the system enters a settle period where quiescent conditions are maintained. These quiescent conditions allow the reactor contents to separate into a clarified effluent upper layer and a lower sludge layer. After separation is complete, the sludge layer rests on the bottom of the reactor and the clarified effluent layer is located above the sludge layer. The effluent layer is subdivided into a lower buffer volume and an upper decant volume.
In a conventional sequencing batch reactor, at the end of the settle period, the decant period begins and the decant volume of the clarified effluent is removed from the reactor. The wastewater level in the decanting reactor falls. The decant volume is normally equal to the volume of influent received during the previous fill period. However, the decant volume and, therefore, the fill volume are limited to a maximum volume based on the dimensions of the reactor. The buffer volume is retained in the reactor during the decant period and provides a buffer zone between the sludge layer and the decant volume to reduce the possibility of sludge uptake during the decanting process.
At the end of the decant period, the reactor typically enters an idle period until each reactor of the system has sequenced through the filling cycle after which influent wastewater is directed back to that reactor and the reactor begins its cycle again with the fill period. The length of certain of these steps has been varied for various reasons and, in particular, to respond to varying influent flow rates.
The sequencing batch reactor process has gained wide popularity due to its unique ability to meet a wide range of advanced treatment standards. In a conventional sequencing batch reactor process, the anoxic time segments inhibit the growth of obligate aerobes (microorganisms that require the presence of free oxygen), including most filamentous microorganisms that do not settle well. The presence of poor settling filamentous microorganisms in the final effluent adversely affects effluent quality. Further, certain microorganisms present in the biomass will utilize the combined oxygen available in nitrates in the intermediate buffer volume of treated wastewater remaining in the reactor after each cycle to uptake soluble carbon in the incoming wastewater under anoxic conditions. These microorganisms convert the soluble carbon and nitrates (NO3—) to energy, nitrogen gas (N2), water, and carbon dioxide (CO2). This reaction is referred to as denitrification, and is required in many treatment facilities discharging to wetlands, or sensitive streams.
True sequencing batch reactor processes also include a react period. In this aerobic cycle segment, no wastewater is received in the reactor, and the mixed liquor is aerated to fully degrade all carbonaceous waste material in the reactor. This step in the operating cycle creates a “famine” condition in the reactor. Simply stated, there is no food remaining for the microorganisms. This “famine” period inhibits the growth of fast growing microorganisms, including filaments, which adversely affect effluent quality.
True sequencing batch reactor processes also include a quiescent settling period during which no influent is received and no effluent is withdrawn. The presence of a quiescent settle period improves liquid solid separation, and thus, effluent quality.
Based on the maximum cycle time, the conventional operating strategy is time based with level overrides. Timers are set for the fill period, the anoxic fill period, the aeration period (aerated fill period plus aerated react period) and the settle period. The fill timer is set for a period equal to the maximum cycle time divided by the number of reactors. For example, in a two reactor system having a six hour maximum cycle time, the fill timer for each reactor would be set at three hours. The influent flow rate required to fill the reactor or, more specifically, the maximum decant volume within the time set on the fill timer, is the design flow rate. The maximum cycle time is generally picked so that the reactor operates a majority of the time at flow rates near or below the design flow rate.
In conventional batch reactor systems the batch volume is generally between 20% and 40% of the reactor's total volume, or depth. This results in a necessary total hydraulic differential through the reactor basins equal to the difference between the maximum mixed liquor level and the minimum mixed liquor level in a reactor.
In conventional batch reactor systems, the varying liquid level in the reactor basins requires that dedicated piping be used to convey compressed air from an air blower system to the air diffusion devices in the reactor basins. Alternately, a complex pressure control system may be used to maintain a constant pressure against which the air blowers pump to prevent air from flowing to an undesired reactor due to a lower liquid level, and hence less back pressure in this reactor.
In conventional batch reactor systems, extremely high influent flows cause periods of simultaneous fill and decant. Since the liquid level must fall from maximum level to minimum level during the decant period, the decant flow rate is generally approximately one and one half time the maximum influent flow rate. Downstream process equipment such as disinfection or filtering systems must be sized to handle these extremely high decant flow rates.
At flow rates above the design flow rate, the time available for the non-filling reactors to go through the various treatment steps becomes a limiting factor in the wastewater treatment strategy. As the influent flow rate increases, the fill time decreases, thereby decreasing the time available for the non-filling reactor(s) to complete all the treatment steps.
Because the time required for the settle period and the decant period is generally constant for full batches and the fill period is determined by the influent flow rate, conventional operating strategies generally compensate for the narrowing time constraints by reducing the idle period until it becomes zero, and then start to reduce the react period. The react period can be initially reduced without reducing the overall aeration time by providing for a longer aerated fill period, although this does reduce anoxic treatment time. Then, as the time constraints narrow, the aerated fill period is continually increased and the react period is decreased. This will reduce the anoxic fill period while maintaining a constant aeration time. Eventually, the react period is eliminated and all of the aeration takes place during the fill period. The anoxic fill period is consequently eliminated. Elimination of the anoxic fill period and the react period (as opposed to an aerobic fill period) is undesirable. Each of these steps is important for the effective decomposition of waste material by microorganisms contained in activated sludge.